The packaging, transportation, and storage of hot melt pressure sensitive adhesives presents a problem to industry because once formed, HMPSA particles tend to agglomerate or "block". HMPSAs are produced in a molten state at high temperatures. From the molten state, the adhesives are cooled to solid form and packaged in a form usable by application equipment. In order to reduce the thermal decomposition of the HMPSA at application temperatures, the melt reservoir of most application equipment is kept small enough to turn over the adhesive rapidly. For this reason, it is necessary to produce blocks, slats, or pellets of HMPSAs which can be manipulated easily by equipment operators and will fit into the openings of the melt reservoirs.
There are two common methods to produce easily handled presentations of HMPSAs. One method is to produce relatively large (0.5-2 kg) blocks of material packaged in strippable containers such as silicone coated paper boxes or plastic films. Another approach is to produce smaller particles such as slats (an extruded ribbon of material cut into individual rectangular shaped pieces weighing 2-10 grams each) or pellets. Pelletized or slatted adhesives must be protected by the use of non-adhesive coatings to prevent the adhesives from blocking.
In order to be effective, an anti-blocking coating must fulfill four basic requirements. First, the coating must be easily and efficiently applied to the adhesive particles. Second, the coating must provide storage stability for the adhesive under normal transportation, storage, and handling conditions. Third, the coating should combine with the adhesive when placed in the applicator. Finally, the coating should not diminish the adhesive properties of the HMPSA.
Materials used for anti-blocking coatings include talc, powdered polyethylene, co-extruded polyethylene films, and polymeric coatings. Powders in general are of limited utility due to the tendency of HMPSAs to flow past the powder coating and block. Additionally, inorganic non-thermoplastic powders such as talc or calcium carbonate are insoluble in the HMPSA and can separate, causing plugging of the applicator. Co-extrusion or "sausage" wrapping of anti-blocking films require costly extra steps in the HMPSA production process. The most desirable types of anti-blocking coatings are those which can be applied to cooling molds or directly to the particles of adhesive.